Real time three-dimensional echocardiography (RT3D echo) is a novel technique that provides non-invasive volumetric imaging of cardiac structures. This study was undertaken to determine the feasibility and accuracy of volume measurements of the ascending aorta with RT3D echo. To this end, 21 patients (age: 54+/-17 years; 13 men and 8 women) with different cardiac conditions underwent RT3D echo and multiphase breath- hold cine GRE magnetic resonance imaging (MRI) examinations in random order on the same day. RT3D echo images were acquired from the parasternal window. Images of the ascending aorta were analyzed by independent observers blinded to the results obtained with the other technique. Volume measurements from RT3D echo images were performed with an interactive aided manual tracing technique based on a Silicon Graphics workstation using customized software. The length of the ascending aorta that could be measured from the RT3D echo images ranged from 3.02 to 6.82 cm (mean+/-SD: 4.5+/-1.0). Because the first 3 cm of the ascending aorta could be imaged in every patient, the volume of the proximal 3 cm of the aortic root was compared with the corresponding MRI measurement. The aortic root volumes ranged from 11.0 to 36.9 ml (mean+/-SD: 22.3+/-6.5) by MRI and from 13.1 to 36.2 ml (mean+/-SD: 20.7+/-5.8). A strong correlation was observed between aortic root volume measurements obtained with MRI and with RT3D echo images (r=0.87; y=3.34+0.78x; SEE=2.9 ml). The difference between the two measurements was -1.7+/-3.2 ml. Thus, RT3D echo images of the ascending aorta allow volumetric measurements of the proximal 3 cm of the aortic root in all patients. These measurements strongly correlate with those derived from MRI images and may therefore be useful in the non-invasive evaluation of patients with aortic root disorders. - three-dimensional echocardiography; aortic root - Human Subjects